This invention relates to powder coating compositions and powder slurry coating compositions intended for use in the automotive and/or transportation industries. This application is a Continuation-in-part of U.S. patent application Ser. No. 09/741,511, filed Dec. 19, 2000, now U.S. Pat. No. 6,541,594.
Powder coating and powder slurry compositions provide many of the performance properties of traditional solvent based coating compositions without the complications of liquid organic solvents. Powder coating compositions are applied as dry, finely divided particles. Powder slurry compositions typically include a first component that is in solid particulate form, i.e., a powder coating, and a second component that is liquid. The first component is dispersed in the second component to provide a slurry that can be applied using conventional spray application equipment and techniques.
In particular, the application and cure of powder coatings and powder slurry coatings results in significantly reduced emissions of volatile organic compounds (VOC""s). Powder coatings and powder slurry coatings are also more amenable to recycling than traditional liquid coatings. As a result, powder coatings and powder slurry coatings have the potential for environmental and financial advantages.
However, powder coating compositions and powder slurry coating compositions can present manufacturing, performance and/or application challenges.
For example, the manufacture and application of powder coatings requires the use of resins that are solid at approximately room temperature. The prior art has made numerous attempts to translate the manufacturing, performance and/or application advantages of liquid resins to solid resins suitable for use in powder coatings.
While progress has been made, there still exists a need for a powder coating resin having an advantageous blend of properties with respect to manufacturing, application and/or performance attributes.
Automotive OEM coating applications require films having optimum smoothness, distinctness of image (DOI) and gloss. These performance requirements present challenges to the formulator of powder coatings and powder slurry coatings containing particulate components. The individual particles must flow and level out to the desired level of smoothness in a short time and temperature window, i.e., between the Tg of the particulate resins and the crosslinking temperature. Individual particles that have not reached the desired level of smoothness and flow by the time that crosslinking temperatures are reached are permanently stopped from any further flow and leveling by the crosslinking and curing of the film.
Traditionally, higher films builds have sometimes been required to obtain finished films having a satisfactorily smooth appearance and necessary distinctness of image (DOI). However, the use of more coating composition as a corrective action is disadvantageous with respect to cost and initial introduction concerns.
In addition, powder coatings and powder slurry coating compositions are often vulnerable to gassing and popping defects. Such defects are believed to result from the upward passage of volatile gases from the curing film. Such gases may be the volatile products of crosslinking reactions. In the case of powder slurry coatings, the gases can also be the volatilization of the solvent used. In most cases this will be water together with other organic solvents. In the case of non-aqueous powder slurry coatings, it will be primarily organic solvents. Finally, these defects can be the result of trapped air escaping the coating while the film is curing. The frequency of pop defects may range from the intermittent to large tightly packed masses of hundreds of individual defects. In either case, the presence of these gassing or popping defects often renders the cured surface commercially unacceptable.
Traditional powder coating compositions, especially those used in clearcoat and topcoat applications, often utilize epoxy acid resin systems. Such systems often sacrifice scratch and mar resistance in order to obtain acceptable water resistance, chip resistance and etch resistance.
The processing of powder coating compositions and powder slurry compositions containing powder coatings typically requires the application of initial temperatures above the Tg of the powder resin(s), followed by the application of temperatures sufficient to affect crosslinking if the coating is thermosetting. Thus, high cure schedules are often required. Application concerns and energy costs encourage the use of powder compositions having lower Tg resins. The use of lower Tg resins facilitates more rapid flow, improved appearance and lower cure temperatures.
However, storage and manufacturing problems can result when powder coating compositions having lower Tg resins are utilized. Manufacturing is particularly challenging, especially with commonly used acid epoxy resin systems. Agglomeration of the solid particulate component during storage may be observed.
Inferior film adhesion and/or cracking of the applied powder coating film during curing may occur, for example, when the powder coating is applied over a previously applied coating. These negative performance characteristics can be magnified by higher film builds and poor storage stability of the powder coating composition.
Finally, finished films obtained from powder coating compositions can exhibit variable gloss and/or inferior performance characteristics, particularly when balancing scratch and mar resistance against water resistance, chip resistance and etch resistance. It is often difficult to achieve an acceptable balance of properties among these characteristics, especially with traditional acid epoxy powder/powder coating resin systems employed as clearcoats or pigmented topcoats.
Thus, it is desirable to provide improved powder coating compositions wherein the foregoing disadvantages are either eliminated or are minimized. Prior art attempts hereto have been unsuccessful.
In particular, it would be desirable to provide a curable coating composition containing a particulate component, i.e., either a powder coating composition or a powder slurry coating composition, that can obtain smooth, level cured films having optimum DOI without negatively affecting any other desirable application or finished film properties.
It would be also be desirable to provide a curable coating composition containing a particulate component, i.e., either a powder coating composition or a powder slurry coating composition, that is more resistant to gassing or popping defects without any loss of other desirable application or finished film properties.
These and other objects of the invention are met with the instant invention.
The curable coating compositions of the invention comprise a particulate component having at least one binder component (a) which is a solid at 75xc2x0 F./24xc2x0 C. and an additive component (b). The curable coating compositions also comprise a crosslinking component (c) that is reactive with binder component (a).
Additive component (b) has from 12 to 72 carbon atoms, is substantially free of any heteroatoms, is not a crystalline solid at 75xc2x0 F./24xc2x0 C. and comprises a mixture of two or more structures selected from the group consisting of aliphatic structures for additive component (b), aromatic-containing structures for additive component (b), cycloaliphatic-containing structures for additive component (b), and mixtures thereof, wherein at least one of the two or more structures is either a cycloaliphatic-containing structure or an aromatic-containing structure.
The crosslinking component (c) may be in the particulate component or may be in a liquid component into which the particulate component is dispersed.